Injection molding resin

Abstract

Injection molded plastic parts (such as containers for ice cream or margarine, lids for the containers and crates) are made from a polyethylene resin having a controlled but narrow molecular weight distribution and a uniform comonomer distribution. The combination of narrow molecular weight distribution and uniform comonomer distribution allows the parts to be more easily molded whilst still maintaining a surprisingly high level of physical properties in the finished parts. The polyethylene resin is prepared in a dual reactor polymerization process.

What is claimed is:  
     
         1 . An injection molded part made from polyethylene copolymer characterized in that said polyethylene copolymer is polymerized in a polymerization process having at least two stirred polymerization reactors arranged in series and operating at different polymerization temperatures.  
     
     
         2 . The part according to    claim 1    wherein said polymerization process is a solution polymerization process which operates at a temperature of from 120° C. to 300° C.  
     
     
         3 . The process according to    claim 2    wherein said polyethylene copolymer is a copolymer of ethylene and at least one alpha olefin selected from butene, hexene and octene.  
     
     
         4 . The process according to    claim 3    wherein each of said at least two stirred polymerization reactors has independent feed streams for monomer and polymerization catalyst.  
     
     
         5 . The process according to    claim 4    wherein said polymerization catalyst comprises at least one group 4 metal component wherein said group 4 metal is selected from titanium, hafnium and zirconium; and at least one group 13 metal component wherein said group 13 metal is selected from aluminum and boron.  
     
     
         6 . The process according to    claim 5    wherein said group 4 metal is titanium.  
     
     
         7 . The process according to    claim 6    wherein each of said independent feed streams for said monomer is operated such that said monomer is added to each of said polymerization reactors at a temperature of at least 20° C. lower than the polymerization temperature of said polymerization reactors.  
     
     
         8 . The process according to    claim 7    wherein said injection molded part is a container having a volume of less than 4 liters and wherein said polyethylene is further characterized by having: 
 a) a density of from 0.940 to 0.960 grams per cubic centimeter; and  
 b) a melt index, I 2,  as determined by ASTM standard D1238, condition 190° C./2.16 kg of from 20 to 100 grams per 10 minutes.  
 
     
     
         9 . The process according to    claim 7    wherein said injection molded part is a container lid and wherein said polyethylene is further characterized by having: 
 a) a density of from 0.920 to 0.940 grams per cubic centimeter; and  
 b) a melt index, I 2,  as determined by ASTM standard D1238, condition 190° C./2.16 kg of from 50 to 100 grams per 10 minutes.  
 
     
     
         10 . The process according to    claim 7    wherein said injection molded part is a pail or crate having a volume of greater than 10 liters and wherein said polyethylene is further characterized by having: 
 a) a density of from 0.940 to 0.960 grams per cubic centimeter; and  
 b) a melt index, I 2,  as determined by ASTM standard D1238, condition 190° C./2.16 kg of from 5 to 15 grams per 10 minutes.